Lithium is the metal with the smallest atomic radius. This unique position in the periodic table of elements entails many advantageous properties. Its smallness allows lithium to intercalate easily into the network of many solids. Upon intercalation lithium easily loses its outer electron to the network thereby altering the electronic structure of the solid it invades. The altered electronic structure expresses itself in changes of e.g. electron conductivity or optical absorption. Over and above, in many networks the process of intercalation is reversible in that lithium can be extracted—or de-intercalated—out of the solid's network by application of e.g. an external electric field. This property is advantageously used in Li-ion batteries as they are nowadays widely used in high-end applications such as lap top computers and cell phones. The change in optical absorption can be conveniently used to build electrochromic devices such as in switchable mirrors or glass panes with adaptable transmission. Although other elements such as hydrogen and sodium can also be used in such applications they suffer from disadvantages such as hydrogen being a gas and sodium being already quite large to enter a solid state network.
In order for such ion switchable devices to function, at least two types of material must be present (not taking into account the other trivial necessities as electrical contacting and encapsulating layers). Firstly—as already mentioned—an electrode must be present in which the lithium can easily enter and also, at least partly, exit the lattice (in case of a crystalline material) or network (in case of an amorphous solid). Such lithium activatable electrodes usually consist of compounds comprising oxides of transition. A notable example is e.g. tungsten oxide W(+6)O3 that is transparent for visual wavelengths. Upon intercalation of Li, WO3 darkens with a bluish colour. Tungsten oxide bleaches upon lithium de-intercalation. Tungsten oxide is the most widely used electrode material in electrochromic devices. Another example of an electrode material is lithium cobaltate LiCoO2 that is used as the cathode material of secondary batteries. It acts as a reservoir for lithium charging in secondary batteries.
Secondly, the lithium must be fed to the network as an ion, as otherwise it will not intercalate into the network. This can be accomplished by providing an ion conductor in close contact with the ion-switching electrode. Normally this is done in a planar configuration as this makes the contacting of both layers easy. Such an ion conductor conducts the Li+-ions well while blocking the electrons.
By applying a voltage difference over the electrode/ion-conductor stack by means of a power source, Li+-ions will be extracted out of the electrode, percolate through the ion conductor and be neutralised at the contact where they recombine with the electrons provided by the power source. This is of course when the ion conductor is negatively biased with respect the electrode contact. As a consequence a layer of atomic lithium will deposit between the ion conductor and the ion-conductor contact. Optionally, a counter electrode may be provided between ion-conductor and its contact. This counter electrode acts as a reservoir, collecting and neutralising the lithium ions. While for a battery or a reflective electrochromic device, such a counter electrode is not absolutely necessary, it is needed in case of a transmissive electrochromic device as the metallic lithium would inhibit the trespassing of light. Mutatis mutandis, when now the voltages are reversed, lithium atoms will be ionised at the lithium-ion conductor interface, travel through the ion-conductor and intercalate into the electrode.
From the above it will be clear that somewhere in the manufacturing process, lithium will have to be introduced into the device in order to make it work. However, this input of lithium turns out to be difficult due to a number of reasons. Firstly, when the lithium has reduced the electrode, the electrode becomes vulnerable to oxidation. Oxidation of the lithium in the electrode makes it ineffective in the use of the device. As many ion-conductors are indeed oxides, direct sputtering onto a lithiated electrode cannot be done in an oxidising atmosphere without special precautions. Secondly, the amount of lithium introduced into the device must be carefully controlled. Too much lithium readily leads to overcharging of the electrode. Over time, the following methods for introducing lithium into the device have been proposed:                A. Lithiation by sputtering or evaporation of metallic lithium directly onto the electrode        B. Lithiation by sputtering or evaporating lithium containing compounds directly onto the electrode        C. Lithiation by electrochemical means directly onto the electrode        D. Lithiation after the ion conductive layer has been deposited onto the electrodeOne way of implementing method ‘A’ has been described in U.S. Pat. No. 5,830,336 and U.S. Pat. No. 6,039,850. Lithium is sputtered away from a metallic lithium target onto the electrode by means of an argon plasma that is magnetically confined in the vicinity of the target. The target is preferably AC (10 to 100 kHz, US '336) or pulsed DC powered (US '850). This method results in a well controlled way of adding lithium to the electrode as well as being compatible with the previous low pressure atmosphere sputtering of the electrode (in case WO3, pressure can remain low, no exposure to atmosphere). However the method also has drawbacks: the handling and sputtering of metallic lithium targets is not straightforward due to the very oxidising nature of Li. Also the deposition step of the electrode—generally performed in an oxidising atmosphere—must be well separated from the lithiation step in order to prevent oxidation of the target and electrode. Notable is that lithiation has to be performed as a separate process step.        
Method ‘B’ uses targets of lithium containing compounds that either do not form a compound layer on the electrode (and only leave the Li behind) or that form a compound or alloy that later on can be advantageously used. The former method is e.g. described in U.S. Pat. No. 5,288,381 wherein Li2CO3 in an argon atmosphere is sputtered on an electrode material. The Li2CO3 decomposes during sputtering to yield lithium metal and volatile gas components that are removed by pumping. The latter method—of which U.S. Pat. No. 5,019,420 is an illustration—alloys of e.g. LiSi are sputtered onto the electrode until sufficient Li intercalation is achieved. The electrode is after this step coated with a Li depleted Si layer. Subsequently, the sample is submitted to an oxidising atmosphere, where an oxide containing ion-conductor is grown. In either case, method ‘B’ necessitates the insertion of a supplementary processing step between electrode deposition and ion conductor deposition.
Method ‘C’—illustrated in U.S. Pat. No. 5,370,775—necessitates the exposure of the electrode to atmospheric circumstances. Indeed once the electrode has been deposited (be it by sputtering or by any other known technique in the art such as wet chemical coating), it has to be transferred into an electrolytic chamber containing a Li electrode or a Li containing electrolyte. While again a separate lithiation step is needed, it now even has to be performed in a different environment—a fluid—then the environment wherein the electrode has been deposited.
Method ‘D’ is illustrated in U.S. Pat. No. 6,094,292 wherein lithiation is done after the deposition of the electrolyte—in this case lithiumphosphorousoxynitride—through a separate lithiation step (cfr. FIG. 16, Example 4, how lithiation is performed is not revealed). In other embodiments (FIG. 10, FIG. 12, Example 1 and 2) of this patent, the lithium is provided through the deposition of LiCoO2 on top of the electrolyte, as a counterelectrode. Lithiumcobaltate is a preferred choice of material as it is transparent when oxidised and absorbing when reduced, hence is complementary to the electrode. Again, first the electrolyte is deposited followed by a separate step wherein lithium is fed into the device.